1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copier, a printer, etc. and more particularly to an improvement of a double-sided image forming apparatus capable of forming a double-sided image on the front and back surfaces of a recording medium.
2. Description of the Related Art
A related double-sided image forming apparatus capable of forming a double sided image generally adopts a system in which after a first image formed on a latent image carrier such as a photosensitive body is transferred and fixed on the first surface of a sheet of paper, the sheet is inverted and fed again and a second image formed on the latent image carrier is transferred and fixed on the second surface of the sheet.
This system, however, has the following defects. Since the sheet is passed twice through an image forming unit, productivity per a sheet of paper is 1/2 or less as slow as that in single-sided recording. Further, since the sheet curls at the first time of fixing, during the second time of transferring and fixing, poor transfer, paper wrinkle, and jam on a carrying passage are likely to occur. In addition, noise occurs when the sheet is inverted and fed again.
In order to solve such a problem, a double-sided image forming apparatus has been proposed in which a first toner image (first image) and a second toner image (second image) are respectively formed on two photosensitive bodies located oppositely, and thereafter the first image and second image on the respective photosensitive bodies are transferred simultaneously on both sides of a sheet of paper (e.g. JP-A-63-63057 and JP-A-2-259670).
Further, another double sided-image forming apparatus has also been proposed in which a single photosensitive body (e.g. photosensitive drum) carrying a first image and a second image and an intermediate transfer belt for holding the first image once are provided, a first transfer unit for transferring the respective images on the photosensitive body on the intermediate transfer belt or sheet of paper is arranged at a first transfer area; and a second transferring unit for transferring the first image on the intermediate transfer belt on the sheet and a sheet separator are arranged at the sheet discharge terminal of the intermediate transfer belt (e.g. JP-A-1-209470).
Further, the applicant of this application has already proposed a double-sided image forming apparatus in which with a first and second image forming units each composed of a photosensitive body and an intermediate transfer belt being provided so as to correspond to both surfaces of a sheet of paper, respectively, the images onto the photosensitive bodies are transferred primarily on the corresponding intermediate transfer belts, respectively, and the first and second images on the intermediate transfer belts are transferred secondarily on both surfaces of the sheet, respectively (JP-A-8-108449).
These systems, which is capable of double-sided recording by passing the sheet only once -through the image forming unit, can enhance the productivity while solving the above problem.
However, it has been found that the above double-sided image forming apparatus encounters the following technical problem. Even if the first and second image forming units are arranged symmetrically, minute speed variation due to a load change in the photosensitive body and intermediate transfer belt or the like, makes a difference in the tip positions of the first and second images for the sheet and magnification factor.
Such a technical problem is particularly remarkable in a system of using a belt-shaped photosensitive body or intermediate transfer belt since the above minute speed variation is affected by extension of the belt-shaped photosensitive body and intermediate transfer belt in addition to their load variation
It has been found that a full-color image recording system further encounters a technical problem that color discrepancy occurs among color components (e.g. yellow (Y), magenta (M), cyan (C) and black (K)), so that the image quality is greatly impaired.